1. Field of the Invention
This invention relates to a conveyor system in which an optoelectronic reader unit having an energy beam emitter and beam sensor coacts with reflector means of a transport vehicle mounted code device to initiate operation of a spur line switch.
2. Description of the Prior Art
In the typical truck tow conveyor system, the truck transport vehicles move along a main line to spur line switches that can be operated to divert particular vehicles to respective spur lines. Each transport vehicle is provided with a suitable selectivity or coding device for selectively giving a signal to a particular optoelectronic reader unit coupled to a particular spur line switch so that selected vehicles can be diverted to a preselected spur line.
The U.S. Pat. application Ser. No. 301,025 of H. M. Swartz filed Oct. 26, 1972 (now abandoned) and the U.S. Pat. application Ser. No. 302,374 of C. A. Rosenburger, Jr., filed Oct. 30, 1972 (now U.S. Pat. No. 3,822,646 issued July 9, 1974), disclose truck tow conveyor systems employing coding systems for controlling the movement of transport vehicles or trucks along a main line and selected branch lines. The coding system shown in the application of H. M. Swartz is comprised of a coding device on each movable transport vehicle. Each device includes a code element positionable in any of a plurality of code positions and having a reflector means. Optoelectronic reader units, located in the vicinity of respective spur line switches, each employ a detector unit that has an energy beam emitter casting a vertically oriented energy beam upward through an opening in a cover in a floor into the path of the coding element of particular conveyor vehicles. Each detector unit also has a scanner or sensor device to receive reflected beam energy when the reflector means of a coding device on a vehicle intercepts the beam cast by the associated emitter to reflect the beam toward the sensor device. When the intensity of the reflected beam exceeds a predetermined threshold intensity, the sensor initiates operation of a spur line switch to divert the vehicle having the reflector means onto a spur line.
The reader units shown in the Swartz application include detector units that are located directly beneath an opening in the overlying cover in the floor and are oriented with their optical window elements facing upward. Dust, dirt and other matter may fall through the cover openings and deposit on the optical window elements of the detector units. Matter, such as debris, that gathers on a unit can cover the optical windows of its beam emitter and beam receiver to reduce the intensity of the beam impinging on the receiver or to actually obstruct the beam so that it can not impinge on the receiver. After a short or long period of time enough matter may gather on the optical windows to reduce the intensity of the beam impinging on the receiver (after reflection of the beam by a reflector means of a particular vehicle) to a level that is less than a threshold level which must be reached before resultant actuation of an associated spur line switch can be achieved. Thus, a vehicle carrying a properly positioned code element may approach a spur line switch bringing the code element into optical alignment with an associated reader unit and yet not be diverted by the switch to the associated spur line simply because the intensity of the reflected beam is below the threshold level.